1. Field of the Invention
The present invention relates to a method and apparatus for growing monocrystalline structures of silicon and like materials from a melt of polycrystalline like material.
2. Description of the Prior Art
In the field of forming monocrystalline structures from a polycrystalline melt of the same material, the Czochralski technique and variations thereof are the most popular. This technique involves the melting of polycrystalline charge such as silicon in a quartz crystal by radio frequency induction heating. Assuming silicon is being processed, the charge is melted at approximately 1420.degree. C. A monocrystalline silicon seed is then lowered into the melt in the crucible while being continually rotated in a clockwise direction. The crucible and its charge is also being rotated in a counterclockwise direction. The seed crystal is slowly withdrawn from the melt until the desired diameter of the pulled monocrystalline structure is obtained. The pull speed is increased to maintain the diameter of the pull desired. This will continue as long as there is melt remaining in the crucible.
Examples of variations on the Czochralski technique can be found in U.S. Pat. Nos. 3,853,489; 3,826,625; 3,759,671; and 3,370,927. U.S. Pat. Nos. 3,759,671 and 3,370,927 do not pull the seed perpendicular to the surface of the melt. The disadvantage of the technique involved in these patents is that the width of the pulled crystal is limited to the width of the surface of the melt and surface tension. The disadvantages of the Czochralski technique generally are that the polysilicon charge is limited in size, the maximum crystal diameter obtainable is six inches, and the oxygen content of the monocrystalline structure pulled from the charge is relatively high due to the use of the quartz crucible.
Another technique used for obtaining monocrystalline material, such as silicon, from a polycrystalline structure is known as float zoning technique. This technique involves the use of a polycrystalline rod of silicon as the charge. This rod is rotated in an inert gas environment in a counterclockwise direction. A monocrystalline seed of silicon is rotated in a clockwise direction. An RF induction heating coil is slowly moved across the length of the polysilicon rod causing it to heat portions of the rod as the coil moves along its length. Initially, a molten button forms on the bottom of the polycrystalline rod. The monocrystalline silicon seed is raised so that its tip is immersed in the molten button of the polycrystalline rod. The RF work coil is engaged to travel upward along the length of the polycrystalline rod at a slow programmed rate causing the molten areas within the rod to pass from one end of the rod to the other. As the molten zone passes through the rotating polysilicon rod, single crystal silicon forms. As each molten portion resolidifies, impurities tend to congregate above the molten zone, thereby leaving the newly formed single crystalline rod to be free of impurities. The advantages to this technique are that extremely low oxygen concentration is experienced in the resulting polycrystalline structure. The disadvantages, however, are that the maximum crystalline diameter attainable is three and one-quarter inches, the cost of a float zone furnace capable of producing this diameter is close to a quarter of a million dollars, and the height of the furnace required is approximately two stories.
Some other techniques that have been employed are exemplified in U.S. Pat. Nos. 3,877,883 and 3,899,304. The U.S. Pat. No. 3,877,883 employs what is generally known as the Bridgeman technique which utilizes a sealed factor that is not capable of producing thin, wide crystals. U.S. Pat. No. 3,899,304 utilizes a technique of a molten material as a support for floating melt thereon. This technique cannot be used to manufacture monocrystalline silicon. A molten support material for a silicon melt is presently not known.
None of these techniques contemplate use of an extrusion mold within a temperature-controlled environment, which permits a monocrystalline body to be formed from a polycrystalline charge in a variety of sizes and shapes as dictated by the size of the charge and the shape and construction of the extrusion mold.